Tank underpressure despite new valve

Hi,

I was allowed to refuel today, fuel consumption has dropped by now from 20l/100km to 17,5l/100km, still not so bulging but well right development direction...

When I opened the tank, I noticed a strong hissing, the tank drew air.

I only changed the tank vent a few days ago. The "Hupen" is still "sporadic" there!?

Lambda probe is also new.

Where should I look next?